Acoustic device



May 12, 1931- P. B. FLANDERS 1,804,759

ACOUSTIC DEVICE Filed April 28, 192s IUI VEN 70H ffm/L H ANoL-Hs Y A 7`TURA/EY lbodiments, and from the y Patented May 12,.-'1931 UNITED STATESPAUL B. FLANDERS, OF EAST ORANGE,

'YORK PATENT oFFicE l NEW JERSEY, ASSIGNOR T BELL TELEIfI-IONELABORATORIES, INCORPORATED, OF NEW YORK, N.

Y., A. CORPORATION OF NEW .ACOUSTIC DEVICE Application filed April l28,1928. Serial No. 273,524.

This invention relates to acoustic devices and particularly to anacoustic attenuator comprising series and shunt impedance elementshaving a negligible reactance component at speech frequencies.

' In accordance with the present invention y there is provided anattenuator comprising series and shunt impedances which have anegligible reactance component at speech fremquencies.` Resistanceelements suitable for use in such an attenuating network are disclosedand claimed in one of my copending applications, Serial No. 273,523,filed of even date herewith; In accordance with o-ne of the severalspecific embodiments of the invention herein shown and described theresistances forming the acoustic attenuator comprise sound passages inthe .form of parallel walled slots less than 0.003 inch in width in adirection substantially perpendicular to the direction. in whichacoustic waves are transmitted therethrough. `The sound passagecomprising a series resistance is so posij tioned that acoustic wavespass therethrough in going from one portion to another ofthe acoustictransmission line of which the at-v tenuator forms a part, and the soundpassage comprising a shunt resistance extends from the acousticvtransmission line to4 the free atmosphere. By the use of very smallseparations of t-he order mentioned above, between the walls of thepassages, a purely resistive character of the impedance is ensured for arange of frequencies wide enough to include all the essentialfrequencies of speech or music. The attenuation produced by the deviceis therefore substantially uniform at all frequencies in this range, andthe device is o therefore suited for the accurate control of soundvolume in an acoustic system, without causing any distortion of thesound intensityfrequency characteristic.

The nature of the invention will be more fully understood from thefollowing detailed description of a selected number of itsemaccompanying drawings of which:

Figs. 1 and 2 are cross-sectional views of one embodiment of theinvention;

Fig. 3 is an impedance diagram corresponding to Fig. l;

Fig. 4 illustrates a invention; and

Figs. 5 and 6 illustrate the adaptation of the device of Fig. 4 to anacoustic transmission system.

In Fig. l the main sound conduit is formed preferred*` form of the bythetubular elements l, and the slots by -pending upon the degree ofattenuation desired. The plugs may be held in concentric relationship bymeans of axial pins. vShunt leakage paths 9 are provided at the ends ofblock 3 in the form of narrow radial crevices between the block and thetubular element-s.

The width of these crevices is determined by the height of a pluralityof pins, such as 5, inserted in the end of the elements 1. Screw caps 6,engaging a thread on the outside of block 3 serve to clamp the tubes land the center block together as a unit. Holes 7 in the ends ofthe screwcaps permit the air leaking through the crevices 9 to escape to theouter atmosphere. A pin 8 insertedv in block 3 and engaging holes in theends of elements vl aids in keeping the assembly in alignment. The crosssection of `the assembly along the line 2 2 is shown in Fig. 2.

Fig. 3 illustrates schematically ,the impedances of the device of Fig.l, the usual electrical conventions for the representation of a wavetransmission system being employed. Conductors l0 and l1 correspond' tothe mai-n acoustic channel formed by tubes 1. Shunt impedances 2ZkJcorrespond to the impedances of leakage paths 9 and series impedance Zarepresents that of the concentric slots 4. The acoustic impedance of anacoustic path is defined as the ratio of the excess pressure intensity,i. e. the wave pressure applied to the path, to the volumetric rate ofdisplacement of airin the path. For a very narrow parallel slot theimpedance is resistive and has substantially the-value given by theequation y TFE Where Z is the impedance, l

d p. is the vcoeiiicient of viscosity of the med isv the width of theslot,l

Zthe len -h in the direction of wave propagation, an v A thecross-sectional area.

Inc. g. s. units p. has the value 1.86 10 for air. Th'e above formulaenables the impedances Za and 2Zb to be computed from the physicaldimensions of the slots, but in applying the formula to the radial paths9,.v it is necessary to make an approximation tothe cross-sectionalarea, since the area in-V creases from the inner end of the path out-.

' wards. It is suilicient to take the area at the mid-section betweenthe inner and the outer end of the path and to assume that the path hasthis uniform area throughoutits whole length. f.

To avoid'wave reflection effects it is .desirf able that the resistancecombination shouldbe designed with-respect to the acoustic line inywhich it vis inserted. The combination represented by the threeimpedances of Fig. 3 may be regarded as a symmetrical section carved outof an infinite line of recurrent structure, comprising seriesimpedancesZ1 and shunt im edances Zh.' The impedance Z., of such a line,if measured at the middle Ifvnow the values of VZ.,L and Z1, are sochosen that ythe impedance Z.. is the same as the Za? (2)A Icharacteristic impedance of the main acoustic line in which the deviceis inserted, then there will be no wave reiection inthe system. t

If the device is tofbe inserted in an acoustic conduit'ofcross-sectional area A0, the acoustic impedance of which is given by c.g. s. (3)

Then to avoid wave reflection the following Further there. are twovcomplete sections, and .the attentuation will'therefore be equal toe'", where T is the attentuation as defined for a single section.

design condition is imposed tion velocity inthe ratio 6", where e is.the

of a shunt branch, i. e., terminated by a shunt' base of lthe naturallogarithms, the second design condition is imposed, that Z i Y 4 Z-bsinh 2 From equations 4 and 5 the simplified design relations follow:

Za=Z sinh T AZa Zb coth lFor the development and further considerationof the foregoing equation reference is `made to the article by Gr. A.Campbell, The pghysical theory of the electric wave filter, ellvSystemTechnical Journal vol. 1, No. 2, November, 1922.

The preferred form of the attenuating device shown in Fig. 4 comprisestwo sections made up of three series impedance elements -r512', 13, and14, and 2 shunt impedance .elements, 15 and 16, assembled together in acarrier tube 17. The. series elements may each consist of a close coiledspiral of metallic ribbon, wound vwith a uniform pitch ,to give theeffect of a very large slot of narrow width. This type of element andthe manner of constructing it are fully described in my aforementionedcopending application. The ribbon units Yare mounted on sup ortingframes 18, 19 and 20, which are assemg carrier tube 17. The shuntresistance eleled within the ments comprise close spaced helices ofmetallic ribbon wound edgewise on open cylindrical frames 21 and 22.This type is also described in m above mentioned copending application.o ,permit the escape of the air traversing the shunt resistances thecarrier tube 17 is provided with a series of peripheral holes 23. Theseveral elements are held inV ositionl in the carrier tube 17 by means oscrewed clampin rings 24.

The formulae given for t e computation of the resistances in the deviceof Fig. 1 may be used in this case also, but the ollowin differencesbetween the two structures ould be noted'. The device of Fig. 3 ends inseries lmpedances and therefore Acorresponds to a a f ortioin-ofaninfinite recurrent line formed y cutting atthe middle points of twoseries branches, instead of by splitting the'slmnt branches. The endbranches should there'- fore have impedances of the reproduced sound.The attenuating Ydo- 13 already Vice is here arranged to be inserted inthe sound conduit between the tone arm 25, and

" the horn 27 of the phonograph. For this purpose one er more completeunits are as-v sembled in a tube 29, forming transverse passagestherethrough, this tube being slidably mounted in a cylindrical casting28, forming a union between the tone arm and the horn. Fig. 5 is atransverse section or" the assembly and illustrates how the variousparts may be mounted together on the turntable platform 20. Fig. o is anexternal end view of the assembly as seen in the section 6 6. ln theassemblyY illustrated a single attenuating unit is shown, the secondtransverse passage in tube being simply an open tube. By sliding thetube 29 in the casting 28 either the open. tube 30 or the attenuatingdevice may be inserted in the sound conduit, giving a choice ci' twodegrees of loudness of the reproduced sounds. Additional degrees ofloudness .may obviously be secured by the use of additional attenuatineunits.

What is claimed is:

l. An acoustic attenuator comprising se ries and shunt impendances atleast one of which is in the form of a sound passage less than 0.003inch wide in a direction substantially perpendicular to the direction inwhich sound wavesare transmitted therethrough.

2. .an acoustic attenuator comprising series and shunt resistances eachoit which comprises a sound passage ot the order of 0.001 inch wide in adirection substantially per? pendicular to the direction in which soundwaves are transmitted therethrough.

3. ln an acoustic transmission line, a network of series and shuntdissipative elements, said series element comprising an opening throughwhich sound waves pass in going from one part of the transmission lineto another, said shunt element comprising an opening through which soundwaves pass out ot the transmission line, these openings being less than0.003 inch wide in a direction substantially perpendicular to thedirection in which sound waves pass therethrough.

4. lln an acoustic transmission line, a network of series and shuntdissipative elements, said series element comprising a plurality ofopenings through which sound waves pass 1n going from one part of thetransmission line to another, said shunt element comprising a pluralityof openings through which sound Waves pass out of the transmission line,each of these openings being of the order of 0.001 inch wide in adirection substantially perpendicular to the -direction in which soundwaves pass therethrough. Y

5. In an acoustic transmission line, a network of series and shuntimpedances, at least one of said impedances comprising a member formedof a thin ribbon wound in the Jform of a spiral, the adjacent turns ofwhich are so closely spaced that the air passage between them has animpedance to the ilow of air substantially free from reactance atspeechfrequencies.

6. ln an acoustic. transmission line, a network of series and shuntimpedances, at least one of said impedances comprising atmember formedof a thin ribbon wound in the form of a helix having the broad surfaceof the ribbon normal to the axis of the helix, the adjacent turns ofwhich are so closely spaced that the air passage between. them has animpedance to the flow of air substantially free from reactance at speechfrequencies.

ln witness whereof,I l hereunto subscribe my namethis 27th day of April,i928.

.PAUL B. FLANDERS.

